A person skilled in the art is familiar with different options for protecting metals such as, e.g., aluminum from corrosion. These options include, for example, the deposition of metallic coatings, anodic oxidation, the formation of chemical conversion layers or the application of paints and/or lacquers, as well as combinations thereof.
In addition to anodic oxidation, chemically produced conversion layers are particularly important with respect to the corrosion protection, in particular, of aluminum or aluminum alloys.
The term “chemically produced conversion layer” or “chemical conversion layer” is familiar to a person skilled in the art and describes non-metallic and usually inorganic layers on a metal surface that are produced in a chemical reaction with the metallic substrate that is typically realized with a treatment solution, particularly an electrolyte solution, wherein the metal surface is passivated due to the formation of the conversion layer.
Chemical conversion layers can also be formed on metal surfaces other than aluminum or aluminum alloy surfaces.
Chromate conversion layers are usually produced with electrolytes containing hexavalent chromium(VI), particularly with solutions containing hexavalent chromium(VI). However, electrolytes containing hexavalent chromium(VI) represent a significant health risk. Due to these circumstances, methods were developed for producing trivalent chromium(III)-based conversion layers that are free of hexavalent chromium(VI) compounds. Electrolytes containing trivalent chromium(III), particularly aqueous solutions containing trivalent chromium(III), are used for forming trivalent chromium(III)-based conversion layers. Suitable electrolytes containing trivalent chromium (III) for the formation of trivalent chromium(III)-based conversion layers are familiar to a person skilled in the art and described, for example, in DE 196 38 176 A1 and WO 2007/134152.
The formation of the chemical conversion layer and the associated corrosion protection properties are defined, among other things, by the wetting and concentration of the active component in the electrolyte solution and the time of exposure on the metal surface. Another requirement is a sufficient mobility of the active component which typically only exists in a liquid electrolyte solution.
The known methods for producing chemical conversion layers have the disadvantage that only a very thin electrolyte film can be applied onto the surface during a local application. However, the conversion coating process should typically result in a sufficiently thick chemical conversion layer and the formation of cracks in this layer should be simultaneously minimized.
Another disadvantage of the known methods for producing chemical conversion layers can be seen in that the proportioning of the electrolyte or the solution is relatively complicated with conventionally used application means such as a paintbrush or a pencil.